Technical Abstract:
Our laboratory collaborates with USDA-ARS in Parlier, CA in developing thermal treatments based on radio frequency (RF) energy for insect control in legumes to meet postharvest phytosanitary regulations for international market. Our current study focuses on lentils and chickpeas that are two important cropes grown in US Pacific Northwest and require chemical fumigation when exporting to India. But it is difficult to artificially infest lentil and chickpea with live insects for treatment validation. Two similar-in-size legumes, mung bean and black-eyed pea, on the other hand, have high infestation rates of insects. It is our desire to infest those legumes with insects and mix with lentils and chickpeas in treatment validation studies. To obtain conservative results, lentil and chickpea should reach slightly higher temperatures than the infested mung bean and black-eyed pea in the same treatment. The specific objectives of this study were to measure the thermal and dielectric properties of mung bean and black-eyed pea at selected moisture contents and to compare with those of lentil and chickpea reported in the literature. The bulk density of four kinds of beans at room temperature was measured by a standard volume method and the specific heat at 20-90°C by differential scanning calorimetry (DSC). The dielectric properties were measured with an open-ended coaxial-line probe connected to an impedance analyzer over the frequency of 10-1800 MHz, temperature of 20-90°C and four moisture levels. Temperature difference between lentil and mung bean or between chickpea and black-eyed pea were determined in a pilot-scale 27 MHz RF unit. The results showed that the dielectric constant and loss factor of mung bean and black-eyed pea increased with increasing moisture content and temperature, which was in good agreement with the trends observed in lentil and chickpea. When subjected to 27 MHz RF heating for 10 min and 6 min, the final temperatures in mung bean and black-eyed pea were 4 °C and 6 °C higher than those in lentil and chickpea at the same treatment conditions when the temperature of the samples was raised to 60 °C. The fast heating rate of mung bean and black-eyed pea in the RF unit could be caused by the differences in both thermal and dielectric properties. Further research is needed to obtain conservative insect mortality and product quality by reducing the moisture contents in mung bean and black-eyed pea and thus their temperatures during RF processing.